Questions
2 questions per paper
Difficulty
Medium
Importance
Fundamental for anatomy and physiology theory exams
Overview
The renal system is responsible for homeostatic regulation of blood composition, fluid balance, and blood pressure through filtration, reabsorption, and secretion. Understanding these processes is critical for university exams as they form the bridge between basic physiology and clinical pathology in renal failure and acid-base disorders.
Glomerular Filtration
Glomerular filtration is the first step in urine formation where blood is filtered through the glomerular capillary wall into Bowman's space. This process is driven by Starling forces, specifically the balance between glomerular hydrostatic pressure and opposing colloidal osmotic pressure.
- Glomerular Filtration Rate (GFR) is typically 125 mL/min.
- Filtration barrier consists of fenestrated endothelium, basement membrane, and podocytes.
- Net Filtration Pressure = (PGC - PBC) - (πGC - πBC).
- Autoregulation via myogenic mechanism and tubuloglomerular feedback.
Tubular Reabsorption
Reabsorption refers to the movement of water and solutes from the tubular lumen back into the peritubular capillaries. This occurs primarily in the proximal convoluted tubule, which is highly specialized for massive bulk transport.
- Proximal tubule reabsorbs ~65% of water and sodium.
- Loop of Henle establishes the corticomedullary osmotic gradient.
- Aldosterone acts on the distal tubule and collecting duct to increase Na+ reabsorption.
- ADH (Vasopressin) regulates water permeability via aquaporin-2 channels.
Acid-Base Balance
The kidneys maintain arterial pH by excreting metabolic acids and reabsorbing bicarbonate buffers. This is a slow-acting but powerful compensation mechanism compared to the immediate respiratory response.
- Proximal tubule reabsorbs filtered HCO3- via carbonic anhydrase.
- Distal tubule secretes H+ ions linked to phosphate buffers.
- Ammonia (NH3) buffering is the primary mechanism for acid excretion.
- Renal compensation for respiratory acidosis takes 3–5 days to reach maximum.
Formula Sheet
GFR = Kf × [(PGC - PBC) - (πGC - πBC)]
Renal Clearance (C) = (Urine Concentration × Urine Flow Rate) / Plasma Concentration
Henderson-Hasselbalch Equation: pH = 6.1 + log([HCO3-] / [0.03 × PCO2])
Exam Tip
Always draw a labeled diagram of the nephron and indicate the specific transport mechanisms (active vs. passive) occurring in each segment to maximize marks.
Common Mistakes
- Confusing the direction of movement between secretion and reabsorption.
- Neglecting the role of Starling forces in favor of simple diffusion when explaining GFR.
- Failing to mention the specific segments of the nephron where hormone action occurs.
More Revision Notes
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